The present invention relates generally to implantable orthopedic prostheses, and more particularly to implantable patellar prostheses for use at the knee joint.
Generally, the human knee joint comprises three bones, the femur, the tibia and the patella, which each have a smooth surface for articulation on an adjacent surface of at least one other bone. At its distal extremity, the femur has an articulation surface with medial and lateral convex condyles separated posteriorly by an intercondylar groove running generally in the anterior-posterior direction. The condyles join at the distal-anterior face of the femur to form a patellar surface having a shallow vertical groove as an extension of the intercondylar groove.
The patella includes on its posterior face an articulation surface having a vertical ridge separating medial and lateral convex facets. These facets articulate against the patellar surface of the femur and against the medial and lateral condyles during flexing of the knee joint. The vertical ridge rides within the intercondyler groove to prevent lateral displacement of the patella during flexing of the knee joint.
At its proximal end, the tibia includes an articulation surface having medial and lateral meniscal condyles that articulate against the medial and lateral condyles, respectively, of the femur. The mutually engaging articulation surfaces of the femur and the patella together form the patellofemoral joint and the mutually engaging articulation surfaces of the femur and tibia form the tibiofemoral joint. These two functional joints form the anatomical knee joint.
Because of disease or trauma, all or part of one or more of the articulation surfaces of the knee joint may fail to perform properly. This can lead to a need or desirability for replacement of the defective natural articulation surface with a prosthetic articulation surface of an implantable prosthesis. A range of orthopedic implants is available, including patella prostheses used to replace the natural articulation surface of the patella. Such replacements may be accomplished by surgically resecting the patella to remove the posterior portion of the bone, leaving a planar bony surface to which a patellar prosthesis is affixed. The patellar prosthesis typically has an affixation surface affixed to the resected bony surface of the patella with, for example, bone cement. The prosthesis also typically includes an articulation surface that may be made of a biocompatible synthetic polymer material, such as ultrahigh molecular weight polyethylene.
Proper anatomic function of the knee joint is facilitated by proper placement of the prosthetic articulation surface or surfaces during surgery. If the affixation surface is properly affixed to the bony surface of the patella, the prosthetic articulation surface of the patella tracks the intercondylar groove and anterior patellar surface of the femoral prosthesis. During surgery, it is also important and sometimes difficult to mount the patellar prosthesis with proper angular placement.
It would be desirable to have patellar prosthesis designs that reduced or eliminated problems otherwise associated with medial-lateral or angular misplacement of the patellar prosthesis. It also would be desirable to have patellar prosthesis designs able to better simulate natural movement of the patella that occurs during flexing of the knee joint.
The present invention is generally directed to an implantable patella prosthesis. The prosthesis comprises a base component having a fixation portion that allows the base component to be affixed to patellar bone. The prosthesis also includes an articulation component that moves relative to the base component. Additionally, a coupling mechanism is designed to couple the articulation component and the base component in a manner that permits a more desirable movement of the articulation component relative to the base component during flexing of the human knee joint. This permits an individual to more readily adapt to the prosthetic device and also compensates for medial-lateral and/or angular misplacement of the patellar prosthesis during surgery.